The present invention relates to a light-activated bi-directional thyristor which is switched in response to a light signal.
Generally, a bi-directional thyristor comprises a first thyristor region, in which semiconductor layers of different conductivity types are successively arranged in an order denoted as NPNP, from one face to the other of a semiconductor substrate; and a second thyristor region, in which the semiconductor layers are arranged in an order denoted as PNPN. Of the paired main electrodes in this bi-directional thyristor, the one on the main face of the semiconductor substrate serves as a cathode electrode for the first thyristor region and as an anode electrode for the second thyristor region, as well; while the other which is on the other main face of said semiconductor substrate, serves as an anode electrode for the first thyristor region and, also, as a cathode electrode for the second thyristor region. When a trigger signal is applied to the gate under conditions wherein a voltage of a given polarity is being applied between these main electrodes, one of the thyristor regions is rendered conductive.
Light-activated bi-directional thyristors to which light is applied as a gate trigger signal have long been well known. Such bi-directional thyristors have two areas in which carriers activated by light are found. One of them is a first central junction, reversely biased, in the first thyristor region; and the other is a second central junction, reversely biased, in the second thyristor region. These first and second junctions are located at different depths, when measured from the surface of each of the thyristor regions upon which light is incident. Therefore, light-activated bi-directional thyristors have a drawback, in that the sensitivity of light activation is different at the two thyristor regions.
A light-activated bi-directional thyristor which has overcome the above-mentioned drawback was disclosed in, e.g., Japanese Patent Publication No. 56-28029. Such a thyristor is so arranged that the central junction in one of the thyristor regions is extended to a point near its light-incident surface, a part of the extended central junction is made parallel to its light-incident surface, the end of the extended central junction is formed apart from its light-incident surface, and the end of said extended junction is exposed a groove whose surface is coated with passivation glass.
In the thyristor arranged as described above, the number of carriers generated is substantially the same at the first and second central junctions. However, the distance between the carrier generating area and the N type emitter layer varies considerably at the first and second central junctions, leaving the sensitivity of light activation as yet unbalanced in the first and second thyristor regions.